Typical manufacturing processes for memory hard disk media involve plating a disk substrate with a layer of nickel and polishing the nickel surface to obtain a smooth, flat surface that is suitable for application of a layer of magnetic storage media. The computer industry's requirements for increased storage capacity on hard disk drives has necessitated a substantial increase in areal density (data storage capacity per unit surface area) on the disk media. This demand for higher areal densities has required manufacturing improvements of rigid hard disks, including enhanced plating uniformity, reduced surface roughness after polishing, improved flatness and waviness, reduction of defects such as pits and scratches, and enhanced texturing characteristics. The disks' polishing process is an important factor that influences many of these new requirements.
Significant improvements in the polishing process of nickel-plated hard disks have occurred in recent years. Improvements have been made in the equipment used to polish the disks, allowing for better control of post-polish disk flatness. Enhancements in surface inspection methodology have allowed disk manufacturers to inspect for small surface defects previously undetectable, such as pits and scratches. This technology has led to optimization of polishing parameters to reduce defects. Other advances in the area of consumables include: pads, abrasive slurry, and cleaning chemicals. Demand for additional improvements in disk quality have placed an emphasis on precision and consistency of the consumable materials.
Advancements made in the manufacturing of abrasive polishing slurries in the mid 1990's enabled disk manufacturers to apply two polishing steps in their processes. The first step employs a coarse abrasive slurry (nominal 0.3-0.8 micron diameter) to achieve good planarization and fast stock removal. The second step employs a smaller abrasive (nominal 0.2-0.3 micron diameter) that yields a very smooth (Ra less than 6 .ANG.) surface finish with little or no defects. The two-step polishing concept has become the norm for most disk manufacturers.
As polishing technology has evolved, several disk manufacturers have experienced a severe problem with modern polishing machines, especially in the first step application. High friction in the polishing zone involving polyurethane polishing pads, fiberglass disk carriers and the nickel-plated disks can result in a vibrating machine. This vibration results in a loud stick-slip event that continues throughout the polish cycle. The problem can be so severe that the disk carriers are damaged at the gear teeth, and chips of fiberglass debris contaminate the polishing zone. Scratching of the disks results from the fiberglass debris. In an effort to reduce the tendency to cause scratching, some manufacturers have modified their polishing parameters, such as reducing applied pressure. Generally these modifications result in reduced productivity. Furthermore, the resulting cost of frequently replacing fiberglass carriers is very high.
Vibration of this nature is problematic in polishing hard disks for several reasons: 1) it causes an unpleasant loud noise making it difficult for technicians to work around; 2) it leads to rapid degradation of the fiberglass disk carriers that is costly; 3) it can lead to severe scratching if the disk carriers break apart at the gear teeth locations; 4) it can require the manufacturer to reduce the applied machine pressure that causes low removal rate of the nickel and lower productivity; and 5) it can lead to non-uniform surface features on the polished disk.
In general, slurry manufacturers have modified slurry performance with several additives. For example, H. Thrower, Jr., in U.S. Pat. No. 4,038,048, discloses a lapping gel for sharpening cutting edges of lawnmowers. A carboxy vinyl polymer holds the gel's grinding grit in suspension. In addition, R. Schwen, in U.S. Pat. No. 4,544,377, discloses a grinding, lapping and polishing compound. This polishing compound relies upon 15 to 85 weight percent polymeric glycol of ethylene oxide, propylene oxide or butylene oxide or their mono-ethers as its base compound. These polymers have a molecular weight between 500 and 25,000.
In addition, slurry manufactures have altered suspensions with additional additives to improve polishing rates. For example, Okajima et al., in U.S. Pat. No. 4,956,015, disclose a polishing composition that contains water, .alpha.-alumina and boehmite. The boehmite affects the water-based dispersion and improves grinding properties. Furthermore, Yamada et al., in U.S. Pat. No. 5,366,542, disclose a polishing composition containing water, alumina and a chelating agent. The chelating agent alters the alumina dispersion to improve polishing performance.
The most common additions however are cellulose thickeners and surfactants. Cellulose-based thickeners result in an increase in the slurry viscosity. Cellulose-based thickeners can improve lubricity of a polishing slurry. These slurries however require excessive amounts of thickeners to significantly affect disk vibration. And these thickeners can form an undesirable buildup of slurry in the pores of the polishing pads. Surfactants (anionic, cationic and nonionic) also increase lubricity. These additions however, can lead to excessive foaming of the slurry in the distribution system. Since this foaming can reduce removal rate, it is undesirable. In some cases, disk manufacturers have sought to reduce vibration by simply reducing machine pressure in the polishing zone.
It is an object of the invention to reduce polishing machine vibration for eliminating loud noises associated with these machines.
It is a further object of the invention to increase the life of fiberglass carriers.
It is a further object of the invention to reduce contamination in the polishing zone.
It is a further object of the invention to reduce the buildup of slurry and debris in the pads.
It is a further object of this invention to polish with higher pressures to achieve faster polishing rates.